The present invention is directed to a modified porous glass diaphragm in the form of capillary diaphragms for use in a device for diafiltration which device is particularly useful as an artificial kidney.
The expression "diafiltration" relates to a diaphragm or membrane process used for rapid and preventive segregation or reconcentration of low molecular substance from high molecular substances in a solution without a phase change. Such a process also enables fractionating separation. The driving force in a diafiltration device is essentially the pressure differential across the diaphragm or membrane. Because of the use of this driving force, the process of diafiltration differs from dialysis, which uses the concentration gradient as its driving force.
The advantages of diafiltration lie in better process control and in the definite separation of molecules. Hitherto, diafiltration has been mainly implemented by means of polymer membranes or diaphragms of varying plastic materials. These diaphragms have considerable disadvantages in spite of their good functional efficiency. One of the main disadvantages in the poor adaptability of the diaphragm material to certain application problems that occur in using the device. In particular, active molecules, which are especially present in biological solutions, are prone to build up by adhesion or absorption on the surface of the diaphragm and thus reduce the permeability performance of the diaphragm. Other problems reside in the non-universal resistance of the diaphragm material to organic solution reagents, in the relatively poor pressure stability, which results in structural changes at high system pressures and in the low temperature stability, which will, for instance, exclude part of the usual sterilization procedures. As a result of this poor pressure stability and poor chemical resistance, cleaning procedures by reverse flushing in which organic solvents or other inorganic acids or alkalies are used, cannot be carried out particularly in the case of asymmetrical diaphragms.
The design of the device is a decisive factor in medical applications which use a diaphragm for filtering body fluids. A flat plastic material diaphragm requires a greater volume then capillaries in relation to a given diaphragm exchange surface. The greater structural volume is usually combined with greater hollow volume or dead spaces so that more body fluid is required to ensure the diafiltration. Owing to differing flow properties at the surface of the flat diaphragm, there is a tendency of blocking the diaphragm in the slower flow areas.